Silicon-based chemicals are used in a wide variety of applications, such as in biocides, stain- and dirt-resistant polymers for carpets, advanced ceramics for aerospace applications and electronic components. The market for silica and other silicon-containing materials amounts to several billion dollars per year.
One important aspect of this market, not immediately evident even to a first-hand observer, is the fact that all silicon-based materials beyond sand are produced by primitive ceramics processing technologies that: (1) add considerable cost to the typical product: (2) limit the scope of applications, and (3) offer limited opportunity for growth because of the maturity of the process.
Silicon products may be derived from the carbothermal reduction of silica to silicon metal: ##STR3## The resulting metallurgical grade silicon (90-98% purity) must then undergo further processing to make other products. For example, to make many of the industrially useful (high purity) forms of silica (e.g., fumed or electronics grade silica), it is necessary to first react the Si metal produced in reaction (1) with Cl.sub.2 or HCl to make SiCl.sub.4 which can then be burned (e.g., reaction 4): EQU Si+2Cl.sub.2 .fwdarw.SiCl.sub.4 ( 2) EQU Si+HCl.fwdarw.HSiCl.sub.3 +SiCl.sub.4 ( 3) EQU SiCl.sub.4 +H.sub.2 O+O.sub.2 .fwdarw.SiO.sub.2 +HCl+HClO.sub.x( 4)
Carbothermal reduction requires high heat and specialized equipment. The result is an energy and equipment intensive process. Reaction of silicon with chlorine or HCl also requires specialized, expensive equipment to deal with toxic and corrosive materials. Despite these considerable drawbacks, because the basic technology was developed late in the last century and early in this century, all of the processing problems have been worked out. This, coupled with economies of scale, makes this approach to the production of fumed and electronics grade silica commercially successful.
Similar problems pervade the alumina and aluminum chemicals technologies. Indeed, these technologies are even more expensive and complex because of the need to electrolytically reduce molten alumina/cryolite melts to form aluminum, the source of most aluminum chemicals and many aluminum containing ceramics.
The production of silicon-based chemicals follows somewhat similar chemistry. Most silicone polymers derive from the "Direct Process": ##STR4##
This simple reaction only works well when RCl is MeCl or PhCl. When it is MeCl, the major product is Me.sub.2 SiCl.sub.2, which is hydrolyzed and polymerized to give polydimethylsiloxane, the basic silicone polymer: ##STR5##
The above reactions, when coupled with standard organic chemistry reactions, some special derivatives and processing procedures, provide the basis for the major portion of the silicone and silicon chemicals industry. It is surprising that there are few, if any, alternate methods for producing silicon-based polymers. If there were, and these new methods provided commercially competitive materials even a fraction as successful as the silicone polymers, the rewards would be exceptional. Preferably, these new methods should also involve an inexpensive and readily available starting material. In view of this, silica is an attractive starting material for producing silicon-containing species, such as those described above.
Silica, SiO.sub.2, is the most common material found in nature. As sand, it is a basic ingredient in building materials, the manufacture of low-tech glass products and ceramics. In purer forms, it is used as an abrasive (e.g., toothpaste) and as a drying and texturizing agent in food and food-related products. It is also used in the manufacture of electronic materials and optical products.
Silica is a feedstock material used for the manufacture of silicon-based chemicals. Synthetic routes stemming from the use of silica gel offer the important attribute of being very inexpensive (research grade silica sells for .about.$15/kg or less). Additionally, silica gel is very easy to handle due to its relative nonreactivity. Industrial fused silica sells for less than $1/kg, and can be used here.
On the other hand, because of its low reactivity, there are few simple, low-temperature methods of chemically modifying silica. One such method is dissolution in base to give sodium silicate: EQU NaOH+SiO.sub.2 .fwdarw.Na.sub.4 SiO.sub.4 ( 8)
Unfortunately, this reaction has limited application for the formation of useful feedstock chemicals. The recent work of Kenny and Goodwin [Inorganic and Organometallic Polymers, N. Zeldin et al., ACS Symposium Series 360,238, (1987)] on silicic acid esterification provides one successful transformation: ##STR6## Si(OEt).sub.4, currently produced by reaction of EtOH with SiCl.sub.4, reaction (11), is used commercially to form fumed and electronics grade silica. ##STR7## It is also used to form optical glasses and boules for spinning fiber optics.
It has been reported that soluble complexes of silicon can be prepared from silica gel and catechol in water. These reports teach that the reactions of silica with 1,2 aromatic diols lead to the formation of hexacoordinate, monomeric silicon complexes: ##STR8## This approach was modified and refined by Corriu and co-workers by using basic methanol solutions under anhydrous conditions. A. Boudin, et. al., Angew. Chem. Int. Ed. Engl, 25 (5):474-475 (1986). These stable salts could then be alklyated by strong nucleophiles, such as Grignard reagents, to form three (and frequently four) new silicon-carbon bonds: ##STR9##
The problem with this approach is that the catechol complex, tris(1,2-dihydroxobenzoato) siliconate, is relatively expensive and can only be modified under forcing conditions using expensive reagents such as LiAlH.sub.4, RMgBr, or RLi and the products are limited to tri or tetrasubstituted silicon. Consequently, its large scale utility is limited. Furthermore, formation of mono-and dialkyl derivatives was not possible.
The invention described herein resulted from an exploration into methods of making more reactive complexes of silica using aliphatic 1,2 or 1,3 diols, such as ethylene glycol, instead of catechol. Thus, one aspect of the present invention described in greater detail hereinbelow, involves certail novel silicon complexes that may be formed by a reaction between silica and 1,2 or 1,3 aliphatic diols. These complexes have been determined to contain one or more anionic pentacoordinate silicon atoms.
Previously, pentacoordinate silicon species have been reported. For example, U.S. Pat. No. 3,455,980 discloses pentacoordinate silicon complexes of vicinal aliphatic diols, including ethylene glycol. The disclosure in this patent differs from the present invention, however, in that these prior complexes were not formed from silica but, rather, from a compound of the formula (R'O).sub.4 Si in the presence of excess aliphatic diol and an amine. Also, the structures of the pentacoordinate silicon species disclosed in this patent are different from the structures of those disclosed herein.
U.S. Pat. Nos. 4,632,967, 4,577,003, and 4,447,628 are also directed to pentacoordinate silicates, all of which have structures that are different from those of the present invention.
Generally, the prior art has taught that only monomeric, pentacoordinate silicon complexes derive from monomeric tetracoordinate silicon complexes and only dimeric complexes from dimeric starting materials (always bridged by polyalkyl siloxanes). This is despite forming monomeric pentacoordinate silicon under conditions where sufficient diol is added to form dimeric species.
In an article entitled "Pentacoordinate Silicon Derivatives. IV..sup.1 Alkyl-ammonium Siliconate Salts Derived from Aliphatic 1,2-Diols" [C. L. Frye, J. Am. Chem. Soc. 92:5, 1204-1210 (1970)], there are disclosed silicon-based compounds that are similar to, but structurally different from, those of the present invention.
Some additional publications that may be relevant to the background of the present invention are the following: "Cyclic Pentaoxy Siliconates," R. R. Holmes et al., Phosphorus Sulfur and Silicon and the Related Elements 42:1-13 (1989); "Reaction of Grignard Reagents With Dianionic Hexacoordinated Silicon Complexes: Organosilicon Compounds from Silica Gel," A. Boudin, et. al., Angew. Chem. Int. Ed. Engl, 25 (5):474-475 (1986); "Reaction of Catechol with Colloidal Silica and Silicic Acid in Aqueous Ammonia," D. W. Barnum, Inorganic Chemistry 11 (6):1424-1429 (1972); and "Pentacoordinate Silicon Compounds. V..sup.1a Novel Silatrane Chemistry," C. L. Frye, et al., J. Am. Chem. Soc. 93(25):6805-6811 (1971).
In spite of previous work involving functionalization of silica and other work involving preparation of pentacoordinate silicon complexes, there has remained a need for new and improved ways of producing useful silicon compounds. The present invention provides novel pentacoordinate silicon complexes, methods of preparing them from silica, and processes for converting silica into a variety of useful silicon compounds via these complexes.
In a similar fashion, we find that alumina (Al.sub.2 O.sub.3) can also be converted to soluble chemical complexes by reaction with base in the presence of diols.